Grain-oriented electrical steel sheet is a steel sheet containing not more than 7% Si comprising crystal grains aligned in the {110}<001> orientation. Control of the crystal orientation in the production of such grain-oriented electrical steel sheet is realized utilizing the catastrophic grain growth phenomenon called “secondary recrystallization”.
As one method for controlling this secondary recrystallization, the method of completely dissolving a coarse precipitates at the time of heating a slab before hot rolling, then forming finely precipitate called an “inhibitor” in the hot rolling and the subsequent annealing process is being industrially practiced. With this method, to cause the precipitate to completely dissolve, it is necessary to heat the slab to a high temperature of 1350° C. to 1400° C. or more. This temperature is about 200° C. higher than the slab heating temperature of ordinary steel. A special heating furnace is therefore necessary for this. Further, there are the problems that the amount of the molten scale is large etc.
Therefore, R&D on the production of grain-oriented electrical steel sheet by low temperature slab heating have been carried out.
As the method for production of low temperature slab heating, for example Komatsu et al. disclose the method of using (Al,Si)N formed by nitridation as the inhibitor in Japanese Patent Publication (B2) No. 62-45285. Further, Kobayashi et al. disclose as the method of nitridation at that time the method of nitridation in a strip form after decarburization annealing in Japanese Patent Publication (A) No. 2-77525. The present inventors reported on the behavior of nitrides in the case of nitridation in a strip form in “Materials Science Forum”, 204-206 (1996), pp. 593-598.
Further, the inventors showed that in such a method of production of grain-oriented electrical steel sheet by low temperature slab heating, no inhibitor is formed at the time of decarburization annealing, so adjustment of the primary recrystallized structure in the decarburization annealing is important for the control of secondary recrystallization and that if the coefficient of variation of the distribution of grain size in the primary recrystallized grain structure becomes larger than 0.6 and the grain structure becomes inhomogeneous, the secondary recrystallization becomes unstable in Japanese Patent Publication (B2) No. 8-32929.
Furthermore, the inventors engaged in research on the control factor of secondary recrystallization, that is, the primary recrystallized structure, and inhibitor, and as a result discovered that {411} oriented grains in the primary recrystallized structure have an effect on the preferential growth of the {110}<001> secondary recrystallized grains and showed, in Japanese Patent Publication (A) No. 9-256051, that by adjusting the {111}/{411} ratio of the primary recrystallized texture after decarburization annealing to 3.0 or less, then performing the nitridation to strengthen the inhibitor, it is possible to produce grain-oriented electrical steel sheet high in magnetic flux density industrially stably and showed that as a method for control of the grain structure after primary recrystallization at this time, for example, there is the method of controlling the heating rate in the process of temperature elevation in the decarburizing annealing step to 12° C./s or more.
After this, it was learned that the above method of controlling the heating rate is very effective as a method of controlling the grain structure after primary recrystallization. The inventors proposed, in Japanese Patent Publication (A) No. 2002-60842, the method of rapidly heating the steel sheet in the process of temperature elevation in the decarburization annealing process up to a predetermined temperature in the range from the region of 600° C. or less to 750 to 900° C. by a heating rate of 40° C./s or more so as to control the I{111}/I{411} ratio in the grain structure after decarburization annealing to 3 or less and adjusting the amount of oxygen of the oxidized layer of the steel sheet in the subsequent annealing to 2.3 g/m2 or less to stabilize the secondary recrystallization.
Here, I{111} and I{411} are the ratios of grains with {111} and {411} planes parallel to the sheet surface and show values of diffraction strengths measured at the sheet thickness 1/10 layer by X-ray diffraction measurement.
In the above method, rapid heating up to a predetermined temperature in the range of 750 to 900° C. by a heating rate of 40° C./s or more is necessary. Regarding the heating means for this, modified decarburization annealing facilities using radiant tubes utilizing conventional ordinary radiant heat etc., the method of utilizing lasers or other high energy heat sources, induction heating, electrical heating apparatuses, etc. may be mentioned, but among these heating methods, in particular induction heating is advantageous in the points that it has a high freedom of heating rate, enables heating without contact with the steel sheet, and is relatively easy to install in decarburization annealing furnaces.
In this regard, when using induction heating to heat electrical steel sheets, it is difficult to heat electrical steel sheet to a temperature of the Curie point or more, since the sheets are thin, when the temperature becomes close to the Curie point, the current penetration depth of the eddy current becomes deeper, the eddy current circling the front surface in the strip width direction cross-section is cancelled out at the front and rear, and the eddy current no longer flows.
The Curie point of grain-oriented electrical steel sheet is about 750° C., so even if using induction heating for heating to a temperature up to this, for heating to a temperature above this, it is necessary to use another means to take the place of the induction heating, for example, electrical heating.
However, using another heating means in combination loses the advantage in facilities of use of induction heating. Also, for example, with electrical heating, contact with the steel sheet becomes necessary. There was therefore the problem that the steel sheet was scratched.
For this reason, when the end of the rapid heating region is 750 to 900° C. as shown in Japanese Patent Publication (A) No. 2002-60842, there was the problem that it was not possible to sufficiently enjoy the advantages of induction heating.